En passant

From WikiChess

En passant (which means in passing in French) is a type of capture that can be made by pawns. To be performed, a pawn must have just advanced two squares. This move must have placed it adjacent to a pawn of the opposite color that could have captured it, had it moved one square. The opposite-color pawn moves to the place it could have captured the pawn that just moved, and removes it from the board. En passant must be performed immediately or not at all. Due mostly to its uncommonness, the en passant rule is most often ignored, or even prohibited in some tournaments.